As mobile communication and data transmission become increasingly more widespread, there is also an ever increasing interest in the development of filters and resonators or narrowband applications with high stop band attenuation. Filters for GPS devices (global positioning system) having a 10 MHz bandwidth at 1.57 GHz or resonators for frequency standards are examples of such applications.
A high stop band attenuation is usually achieved by using multistage filters in which, by way of example, frequency-shifted series and shunt resonators are connected up in a so-called “ladder structure”. Although a virtually optimum bandwidth of the passband can be achieved with said multistage filters, a very large number of stages is necessary for a high stop band attenuation (out-of-band rejection) since the typical stop band attenuation per stage in these filters is only approximately 6.8 dB. Therefore, at the present time, multistage filters having a ladder structure and a stop band attenuation of more than 50 dB cannot be produced in practice.
A higher stop band attenuation per filter stage can be achieved using so-called “balanced” filters, which generally have a bridge circuit of frequency-shifted resonators. However, the use of these types of filters is subject to some significant restrictions. Thus, in “balanced” filters, the input and output signals must be present in differential (balanced) fashion. Therefore, systems in which such types of filter are used require either special antennas and preamplifiers or else particular components or assemblies which convert the so-called “single-ended” signals that are usually present into so-called “balanced” signals.
The filters and resonators that are commercially available at the present time for narrowband applications are predominantly ceramic filters or so-called “surface acoustic wave filters”. However, these types of filters can be miniaturized only with difficulty and their production is generally complicated and thus cost-intensive. This makes them unsuitable for use in low price products. Furthermore, these filter structures generally cannot be integrated into the customary processes of semiconductor fabrication.
In addition to surface acoustic wave filters, attempts are also increasingly being made to use so-called “bulk acoustic wave” filters as miniaturized filters and to produce these by means of thin film techniques and substrates. So-called “stacked crystal filters” (SCF) constitute a subgroup of these types of filter. A stacked crystal filter typically comprises two piezoelectric layers and three electrodes. The first piezoelectric layer is arranged between a first, bottom electrode and a second, central electrode, a second piezoelectric layer is arranged between the second, central electrode and a top, third electrode. The central electrode is generally grounded in this case. In order to prevent the acoustic oscillations generated in the piezoelectric layers from propagating in the substrate, the stacked crystal filters may be shielded from the rest of the substrate by acoustic mirrors, for example.
The principle of stacked crystal filters has been known for approximately 40 years, but has been unable to gain general acceptance on an industrial scale in the MHz frequency range since the production of corresponding quartz laminae with center electrodes was not mastered. With the progress in miniaturization technology, in particular the progress in thin-film technology for piezolayers, stacked crystal filters are becoming increasingly attractive again. A corresponding filter for GPS applications is described e.g. in “Stacked Crystal Filters Implemented with Thin Films, K. M. Lakin, G. R. Kline, R. S. Ketcham, J. T. Martin, K. T. McCarron, 43rd Annual Symposium on Frequency Control (1989), pages 536-543”. Further examples of the use of miniaturized stacked crystal filters are described for example in the patent specifications U.S. Pat. Nos. 5,910,756 and 5,872,493. The latter describes, moreover, that a stacked crystal filter can be acoustically shielded from the substrate by means of an upper and lower acoustic mirror.
What is common to all the stacked crystal filters described therein, however, is that their production, on account of their complex construction and the associated high number of layers to be deposited and to be patterned, requires a high process outlay which increases the production costs of the filters.